Capitella sp. I is a bcnthic organism of which normal habitat is coastal polluted sediments. Previous observations clarified that presence of this organism may prevent from further deterioration of the environments. The actual mechanism was assumed to be introduction of oxic seawater from upper water column and oxidation of sulfide to sulfur or sulfate. Bacteria may play an important role as nutritional source for Capitella sp. I and/or degrader of organic pollutants in the environments. The purpose of this investigation was to clarify these contributions. During the course of this study, we developed a new method to culture and observe the behavior of Capitella sp. I directly, by using transparent containcr and solidified agar. In addition, we used the chemical (I NT) which made it possible to visualize and quantity the bacterial respiratory activity. The result showed that Capitella sp. I stimulates the activity of bacteria by making holes in the sediment and introducing oxic seawater. As heterotrophic bacterial activity is assumed to couple to degradation of organic matter, it was conchided that presence and behavior of Capitella sp. I does play important roles to prevent from deterioration. This new technique will be applicable for various research on the interact on between benthic organisms and bacteria.